This application requests funds for purchase of a Siemens Stabilipan 2 300 kVp X-ray generator. This instrument is critically needed by the users involved for research on the effects of ionizing radiation on cellular and subcellular systems as well as in experimental animals. In addition, the machine will be available to any others in our institution who need it for work in genetics, photomedicine, oncology research, etc. Our research is, and for the past 15 years, has been conducted on a 35 year old Picker Vangard X-ray machine. The machine breaks down periodically and is becoming increasingly unstable and unreliable in its operation. This is seriously affecting our ability to do research on NIH-funded projects. The machine has been kept functioning thus far through the efforts and expertise of engineers and physicists in our department since significant company service is no longer available. Spare parts (other than X-ray tubes) are also no longer available from the manufacturer. Limited spare parts have been obtained from similar discarded machines but this source of supply has essentially dried up. Our major users group is composed of NIH-funded members of the Department of Radiation Medicine and Cancer Center of the Massachusetts General Hospital. These investigators conduct research into mechanisms of radioprotection and radiosensitization using cell survival and DNA damage assays, inherent radiosensitivity of cells and human tumors, cellular and tumor effects of combinations of hyperthermia with radiation, and predictive assays for effects of radiation on tumors. Other investigators from the Departments of Radiation Medicine, Dermatology, and Hematology-Oncology and the MGH Cancer Center will also use the machine. Calibration, dosimetry and radiation protection will be carried out by physicists from our department. Routine maintenance of the machine will be done by engineers from our department who will arrange for any future major maintenance with the Siemens Company.